This invention pertains to a control valve for a pressurized fluid-operated system, such as a marine power steering system in which a hydraulic cylinder assembly, including an extendible and retractable rod, provides a steering assist to a pivotable marine drive unit.
A marine power steering system typically includes a hydraulic cylinder assembly having an extendible and retractable rod, and a control valve for controlling the supply and discharge of fluid to and from the cylinder, to control extension and retraction of the cylinder rod. The control valve includes a housing having an internal passage, and a spool member mounted within the internal passage. The housing includes work ports which communicate with opposite ends of the cylinder, and the spool member is provided with grooves for routing fluid to one end or the other of the cylinder upon movement of the spool member within the passage.
The control valve is an open center design in which fluid from the pump is returned to the reservoir when the spool valve is in its neutral position, i.e. when no steering assist is required. In the past, when the spool valve is in its neutral position, the work ports are exposed to the reservoir as fluid from the pump flows from the inlet to the outlet of the housing. Both ends of the cylinder are thus exposed to the reservoir when the spool member is in its neutral position. With this design, it is possible for the power steering system to experience drift as a result of propeller torque or other factors, with the result being that the marine drive unit may move away from its desired position even though no steering impetus is applied to the spool member. The external load on the hydraulic cylinder rod begins to create pressure in one end of the cylinder, but since both ends are connected to the reservoir the fluid is allowed to exit one end of the cylinder and enter the other, thus causing the cylinder to drift from its desired position.
It is an object of the present invention to provide a valve for controlling the flow of fluid in a fluid-operated system, such as a marine power steering system, for preventing the work ports from being exposed to the reservoir when the spool member is in its neutral position while also being used in a following or servo positioning mode. It is a further object of the invention to provide such a valve which is simple in its construction and operation, yet which provides highly advantageous operation of the fluid-operated system.
In accordance with one aspect of the inventions a control valve for a pressurized fluid-operated system, such as a marine power steering system, includes a spool member mounted for sliding movement within the housing internal passage, with the spool member being movable between a neutral position and at least a first working position. The spool member includes structure which cooperates with the housing internal passage for establishing communication between the housing inlet and the work port when the spool member is in its first working position. The spool member structure further establishes communication between the inlet and the outlet when the spool member is in its neutral position, while blocking the work port when the spool member is in its neutral position. In this manner, the work port is not exposed to reservoir fluid when the spool member is in its neutral position, and the resulting detrimental effects such as drift of the marine propulsion unit are prevented. The spool member includes a recess arrangement which functions to establish communication between the inlet and the outlet when the spool member is in its neutral position. A protrusion, such as in the form of a land, is provided on the spool member and is located over the work port when the spool member is in its neutral position. The land blocks entry of fluid into the work port in the neutral position. In a preferred form, the recess arrangement includes a pair of annular grooves, one of which is in communication with the inlet and the other of which is in communication with the outlet. A channel extends between the grooves to establish communication therebetween. The annular grooves and the channel cooperate to define the land, which functions to block the work port when the spool member is in its neutral position.
The invention may advantageously be incorporated into a marine power steering system, which includes a pair of work ports, each of which is located between the inlet and one of a pair of outlets. Structure as summarized in the preceding paragraph is provided on the spool member for each of the work ports.
The invention further contemplates a method of controlling the flow of fluid in a pressurized fluid-operated system, such as a marine power steering system. The method broadly comprises the steps of establishing communication between the pressurized fluid source and the work port when it is desired to operate the system, and blocking communication between the pressurized fluid source and the work port while establishing communication between the pressurized fluid source and the outlet, to provide return flow of fluid from the system to the reservoir, when it is desired not to operate the system. The specifics of the method of the invention are generally according to the above-summarized aspects of the apparatus of the invention.
Various other features, objects and advantages of the invention will be made apparent from the following description taken-together with the drawings.